Multi-wire electrical conductor strands are made in different configurations by many different methods. Each method and each configuration has advantages and disadvantages.
One approach is to form the strand with a central wire surrounded by one or more layers of helically laid wires. The strand is made by twisting the wires of each layer about the central wire with a wire twisting machine. A true concentric strand is one example of a strand made by this method. Each layer of a true concentric strand has a reverse lay and an increased length of lay with respect to the preceding layer. In the case of a nineteen wire conductor strand, two passes are required through the wire twisting machine to make the strand: one pass for a six wire layer having a right hand lay over the central wire; and, a second pass for a twelve wire layer having a left hand lay over the inner layer. The two passes result in low productivity in comparison with machines that apply both layers of wire in a single pass.
A unilay strand is a second example of a conductor strand having helically laid layers disposed about a central wire. Each layer of wire of a unilay strand has the same direction of lay and the same length of lay. Because each layer has the same lay length and lay direction, the strand may be made in a single pass. As a result productivity increases.
Unilay strands are commonly used for 12 and 14 AWG conductor strands formed from nineteen separate wires. These strands are formed of nineteen wires of the same diameter twisted in a concentric pattern to form a hexagon as shown in FIG. 1. The wires may be twisted on either a single twist machine or on a double twist machine as discussed in Krafft, "Single Twist Bunching", WIRE JOURNAL 66 (October 1979). The single twist machine has advantages over double twist machines. Strands made on single twist machines are generally more uniform and of a smaller diameter than those formed on a double twist machine. This occurs because of the difficulty in a double twist machine of controlling the tension of the wires entering the closing die where the helical twist is applied to the wires as the number of wires in the strand is increased to nineteen wires. Double twist stranding machines have the advantage of higher productivity because each rotation of the flyer causes two twists of the wire. Moreover, because of differences between these machines, it is common to find double twist machines that are capable of operating at fifty percent (50%) higher rotational speeds than single twist machines. As a result the output of double twist machines is often greater than three times the output of single twist machines.
Unilay strands formed with nineteen wires of the same diameter, such as the diameter D.sub.5 as shown in FIG. 1, are not without problems. As the six wires of the inner layer and the twelve wires of the outer layer are twisted about the central wire in the same way and in a concentric pattern, a hexagonal cross section is formed. The hexagonal cross section presents three basic problems.
1. Six voids V' formed by the sides of the hexagon are inside the circumscribing circle of insulation. These voids are filled with insulation requiring more insulation per unit length of wire as compared with true concentric stranding. PA1 2. Experience has shown that wires at the corners of the hexagon tend to change position as they pass through circular dies because of difficulty in controlling tension. As the corner wires change position, forces act on the layers of wire and cause the wires of the inner layer to urge the wires of the outer layer to "pop out" resulting in a high strand, intermixing of the wires (i.e. "bird caging") and strand breaking. PA1 3. The wires at the corners of the hexagon tend to backup because of the difficulty in controlling tension in the different layers. Backing up results in intermixing of the wires ("bird cages") and wire breaks during the extrusion of insulation. PA1 4. A unilay strand in the finished condition tends to be more rigid than true concentric strands because the wires of each layer tend to reinforce the wires of the other layers against bending making the wires more difficult to work with than true concentric stranding.
As a result of these concerns, engineers in the conductor wire industry have been seeking to develop a conductor strand which increases the flexibility of unilay strands and which would permit the unilay strands to be made on a double twist machine for a wide variety of gages.